New Species Assemblages Disrupt Obligatory Mutualisms Between Figs and Their Pollinators
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fevo-08-564653 November 16, 2020 Time: 14:24 # 1 ORIGINAL RESEARCH published: 19 November 2020 doi: 10.3389/fevo.2020.564653 New Species Assemblages Disrupt Obligatory Mutualisms Between Figs and Their Pollinators Jared Bernard1*, Kelsey C. Brock2,3,4, Veronica Tonnell5, Seana K. Walsh4, Jonathan P. Wenger5, Dustin Wolkis4 and George D. Weiblen5 1 Plant and Environmental Protection Sciences, University of Hawaii–Manoa, Honolulu, HI, United States, 2 School of Life Sciences, University of Hawaii–Manoa, Honolulu, HI, United States, 3 Kauai Invasive Species Committee, University of Hawaii–Manoa, Kapaa, HI, United States, 4 Department of Science and Conservation, National Tropical Botanical Garden, Kalaheo, HI, United States, 5 Plant and Microbial Biology, University of Minnesota–Twin Cities, Saint Paul, MN, United States The reliance of each fig species on its specific pollinator wasp, and vice versa, is the archetype of both obligatory mutualism and coevolution. Pollinator sharing between host fig species is only known to occur among closely related sympatric species. On the Hawaiian island of Kauai, we gathered syconia from 23 non-native fig species, three of which contained the wasp Pleistodontes imperialis. Of the three fig species, one is the wasp’s natural host, Ficus rubiginosa, and another is its sister species, Ficus watkinsiana, which overlaps in native ranges, although researchers have not Edited by: previously documented pollinator sharing. The third fig species, Ficus rubra, is distant Christopher Irwin Smith, Willamette University, United States to the others both in terms of phylogenetic relationship and native range. We found Reviewed by: viable seeds for all three fig species, whereas species without wasps did not produce Carlos A. Machado, seeds. To investigate similarity between these pollinator-sharing fig species, we collected University of Maryland, College Park, United States morphometric data for syconia of our study fig species. We found that fig species David Althoff, with and without P. imperialis significantly differ based on the orientation of their inner Syracuse University, United States ostiolar bracts. These findings suggest that pollinator sharing among these three fig *Correspondence: species may normally be impeded by pollinator competition in the case of F. watkinsiana, Jared Bernard [email protected] and by geographic distance in the case of F. rubra. This work therefore demonstrates that coevolution depends on interactions within native species assemblages, and that Specialty section: mutualisms can be disrupted in new non-native communities. This article was submitted to Coevolution, Keywords: codiversification, convergent evolution, Ficus, fig wasps, fundamental niche, geographic mosaic a section of the journal theory, host breadth expansion, symbiosis Frontiers in Ecology and Evolution Received: 22 May 2020 Accepted: 29 October 2020 Published: 19 November 2020 INTRODUCTION Citation: Bernard J, Brock KC, Tonnell V, Coevolution is the reciprocal evolution of traits between interacting species (Thompson, 2005) Walsh SK, Wenger JP, Wolkis D and and is thus a major driver of biodiversity, as seen in the vast diversity of insects and flowering Weiblen GD (2020) New Species plants (Ehrlich and Raven, 1964; Lunau, 2004; Wiens et al., 2015). Certain insect groups that Assemblages Disrupt Obligatory Mutualisms Between Figs and Their are closely associated with plants have diversification rates that reflect those of their host plants Pollinators. (Danforth and Ascher, 1999; Janz et al., 2006; Kawakita and Kato, 2009; Shimizu et al., 2015; Front. Ecol. Evol. 8:564653. Winter et al., 2017; McKenna et al., 2019). Coevolution can result in strictly obligatory symbiotic doi: 10.3389/fevo.2020.564653 relationships, including obligatory mutualisms, which involve cospeciation demonstrated by Frontiers in Ecology and Evolution| www.frontiersin.org 1 November 2020| Volume 8| Article 564653 fevo-08-564653 November 16, 2020 Time: 14:24 # 2 Bernard et al. New Species Assemblages Disrupt Mutualisms mirrored phylogenies (Hafner and Page, 1995; Pellmyr and the obligatory mutualism, because the species would no longer Leebens-Mack, 1999; Jousselin et al., 2009; Percy, 2010). interact solely with its unique partner. Coyne and Orr(2004, One of the strongest cases of coevolution is between figs p. 189) assert that cospeciation requires one-to-one obligatory (Moraceae: Ficus) and their pollinator wasps (Chalcidoidea: partnerships, as opposed to codiversification whereby groups are Agaonidae), in which each depends upon the other to complete coevolving but not necessarily by strict relationships (e.g., Winter its life cycle (Ramírez-Benavides, 1970; Weiblen, 2002; Cruaud et al., 2017; McKenna et al., 2019). By this definition, if a fig forms et al., 2012b). In particular, a specialized wasp must go through a new facultative pollinator relationship in a new community, the a minute labyrinthine portal known as an ostiole on a fig’s original relationship will no longer be obligatory and there would closed involuted inflorescence (i.e., syconium or “fig”) to deposit be less pressure to maintain it. eggs into the ovules it can reach and pollinate those it cannot The Hawaiian island of Kauai has the potential to shed light on (Weiblen, 2002; Cook and Rasplus, 2003). To maintain the whether mutualisms will be compromised in a new community, mutualism, the fig emits volatile organic compounds (Grison- having no native figs or fig wasps but ∼45 cultivated non- Pigé et al., 2002; Hossaert-McKey et al., 2010; Souto-Vilarós et al., native fig species. Foresters purposefully introduced three fig 2018) that attract a specific pollinator whose ovipositor length species alongside their obligatory pollinators, and only one of complements its flower styles (Herre et al., 2008). Chemical cues these figs, Ficus microcarpa L.f., is documented as naturalized on the surface of the syconium (Wang et al., 2013) and the size (Timberlake, 1924; Beardsley, 1998). A recent botanical survey of the wasp relative to the ostiole (Liu et al., 2013) likely also play found additional non-native fig species that are potentially in important roles. As a result, this strong relationship is formed not the early phases of naturalizing on the island (detailed herein), by host-switching (Moe and Weiblen, 2010) but by coevolution but without knowledge of the arrival of their natural pollinating of figs and fig wasps that largely results in cospeciation, with one wasps. A fig species theoretically requires the arrival of its specific obligatory pollinator species for each fig species (Anstett et al., pollinator to naturalize, although the restricted life cycle of fig 1997; Lopez-Vaamonde et al., 2001; Cook and Rasplus, 2003; wasps reduces the odds of accidental introductions. We therefore Rønsted et al., 2005; Cruaud et al., 2012b). The interdependency question whether newly establishing fig species could involve of figs and their specific pollinators reinforces the obligatory new relationships with pollinators that are already present. mutualisms, and hybridization is hypothetically rare (Moe and Given Kauai’s unnatural combination of non-native figs from Weiblen, 2012; Yang et al., 2012; Souto-Vilarós et al., 2018). across the world, we furthermore suspect that new instances of However, other studies have exposed inconsistencies in the pollinator sharing could manifest between distantly related figs, cospeciation of figs and their pollinators (Machado et al., 2005; and thus phylogenetic relationship would be unable to explain Cruaud et al., 2012a; Yang et al., 2015). Researchers have them as it could for previously studied examples of pollinator- encountered cases of pollinator sharing, which is the sharing of sharing figs. To explore whether convergent evolution can lead a single pollinator among multiple figs (Wei et al., 2014; Wang to novel pollinator sharing among co-occurring non-native figs, et al., 2016), and co-pollination, whereby a single fig species has we collected syconia from nearly two dozen fig species across multiple pollinators (Compton et al., 2009; Wachi et al., 2016). Kauai, examined any seeds and wasps present, and performed a Nonetheless, such exceptions to cospeciation involve sympatric morphometric analysis. closely related fig species complexes (e.g., Cornille et al., 2012). For the most part, fig species within these complexes maintain genetic distinction, but there are instances of hybridization (Tsai MATERIALS AND METHODS et al., 2015). No work has so far determined whether pollinator sharing or co-pollination can exist across species that are more From January–June 2017, we gathered an average of 24 syconia at distantly related than sympatric complexes. various stages of development from each of 23 fig species, which Importantly, the geographic mosaic theory describes that included five subgenera, 10 sections, and 12 subsections (three of the mechanisms supporting a mutualism evolve in response to the sectional lineages in our study do not contain subsections). interactions within a given species assemblage (i.e., community), Most of Kauai’s fig species have not escaped cultivation and are where natural selection favors traits that strengthen the therefore sparsely distributed on private land, so our collections relationship between partners while excluding other species were opportunistic and often confined to the National Tropical interactions (Thompson, 2005). In other words, coevolution Botanical Garden in Kalaheo. Although Ficus microcarpa is is dependent on the species interactions